Photographic tin amplification process

ABSTRACT

This disclosure relates to photographic amplifying systems comprised of a solution of stannous ions and a separate source of chromous ions as a reducing agent which selectively deposits tin metal on latent metal photographic images to increase the density of the images. The amplifying systems are particularly effective with latent metal images of photographic media comprising a radiation activated photoconductor. Also described are processes for amplifying photographic images.

Wyman July 8, 1975 [54] PHOTOGRAPHIC TIN AMPLIFICATION 3,637,386 1/1972 De Ruig et al 96/60 R PROCESS [75] Inventor: John E. Wyman, Lexington, Mass. Primary EXami'ler-w0fl Louie,

, F'l-l O.B';Rbt. [73] Assignee: Itek Corporation, Lexington, Mass. 3:32:22 zf gg z gz omer 0 er L [22] Filed: Sept. 25, 1972 RltdU.S.A l t Dta C l, e f S j sg I 11 1968 This disclosure relates to photographic amplifying sysf lfi 0 uy terns comprised of a solution of stannous ions and a separate source of chromous ions as a reducing agent U 5 Cl 96/48 96/50 96/60 which selectively deposits tin metal on latent metal 1 17/136 E, 117/2l2, 106/1 photographic images to increase the density of the im- [5 1] lm Cl 603C 5 5/26 ages. The amplifying systems are particularly effective [58] Fieid 96/48 PD R 50 R. with latent metal images of photographic media com- 117/130 f 106/1 prising a radiation activated photoconductor. Also described are processes for amplifying photographic im- 56] References Cited ages UNITED STATES PATENTS 5/1970 Case 96/48 PD 9 Claims, No Drawings 1 PHOTOGRAPHIC TIN AMPLIFICATION PROCESS This is a continuation in part of US. Ser. No, 743,982, filed July ll, l968. now abandoned.

BACKGROUND OF THE INVENTION a. Field of Invention This invention relates to processes and systems for amplifying photographic images.

b. Description of the Prior Art The intensification, i.e. amplification, of latent metal images in the photographic an has long been known and is described in the literature. Metal itensification systems, including those containing copper ion solutions as the source of intensifying metal, have been described in the literature. In general, metal itensifying systems are electroless plating baths from which the reduced metal is plated out rapidly and selectively on a latent metal image. To be effective, and intensifying system must plate the metal on the latent metal image, i.e. the primary metal image, at a rate substantially faster than on the background of the image. It is obvious then that all electroless plating baths cannot be expected to function as intensifying systems.

lntensification of photographic latent metal images has been described at great length with respect to systems based on silver ion which is most commonly used. The use of stannous ion in photographic image intensification has been suggested but operable systems based on stannous ions have been difficult to perfect. The basic problem with stannous ion systems has been the need for reducing agents which achieve the necessary selectivity of deposition of tin metal on the latent metal image at a rapid rate to ensure deposition of tin in gray to black form rather than the usual tone which is characteristic of electrolytically deposited tin.

The use of photographic media comprising radiationactivatable photoconductors for production of reversible latent images is described in British Patent Specification No. 1,043,250. In the patent, the method generally requires the formation on the media of a latent reversible image corresponding to a pattern of activating light, which image can be rendered irreversible by treatment with a redox system which deposits substances in the radiation-struck portions of the media, the deposited substance generally being the reduction product of the reducible component of the redox system. The extent of reduction can be controlled to produce an irreversible latent image which is visible or invisible but which can be intensified by exposure to additional amounts of the selected redox system. For example, when the reversible latent image is contacted with silver ions in the presence of a reducing agent for silver ions, e.g. hydroquinone or equivalent reducing agent. the irreversible image obtained is either fully or partly visible, or alternatively invisible, depending on the amount of silver ions used and the activity of the reducing agent, as recognized by those skilled in the art.

The major consideration in attempting to develop suitable intensifying systems with tin ion as the reducible component is to reduce the overall cost of silver salts with relatively inexpensive stannous salts, or at least reduce the requirements for silver slats in processing exposed photographic media of the type described.

Additionally, in producing a tin imaging system which would be useful in photographic applications, it becomes extremely desirable that the development rate be very rapid, i.e., less than 3 or 4 minutes and preferably, less than l-2 minutes total processing time. Also, it is extremely important that the developer baths be stable to degradation. This is particularly important in machine processing since the same solution should be able to be reused many times and when replenishment of regeneration of a particular developer is necessary, it should be able to be done easily and rapidly.

SUMMARY OF THE INVENTION There have now been discovered photographic image amplification systems comprising a solution of stannous ions and a solution of chromous ions as reducing agent for the stannous ions. This system readily achieves the objectives of rapid processing, very stable developer solutions, and ease of regenerating used up developer baths.

The specified reducing agent provides excellent results in the amplification of latent metal images, i.e. primary metal images, of such metals as silver, gold, copper, tin, mercury or palladium. For the purpose of this disclosure, the expression latent metal image" is intended to embrace invisible metal images as well as partly visible metal images, as is generally understood by those skilled in the art.

The latent metal image is produced by exposure of a medium comprising a radiation-activatable photoconductor to an image pattern of activating radiation after which the medium is contacted with a solution of metal ions to form the primary metal image, e.g. silver ions. The latent metal image is thus formed and, after removal of excess treating solutions, the latent metal image is amplified with the present new systems. Amplification is accomplished, in general, by contacting the latent metal image with a source of stannous ions and then with the source of chromous ions as reducing agent to obtain the amplified photographic image. If desired, the latent metal image can be contacted with a reducing agent for the selected metal ions to render the latent image at least partially visible.

The photographic medium of this invention comprises a visible photographic metal image, the metal of which is substantially tin, preferably formed by contacting the medium with a solution of stannous ions. In a preferred embodiment, the metal image includes minor amounts of a metal catalytic to tin deposition such as silver or copper and a major amount (more than 50% by weight) of tin. Also, in the preferred embodiments, the final metal image will also contain minor amounts of photosensitive material which have been made catalytic to physical development such as by exposure, e.g., exposed silver halide, zinc oxide, or titanium dioxide. in the preferred copy media wherein the photosensitive material is deposited in a binder, the final tin image contains binder and photosensitive ma terial in addition to the tin and in most cases minor amounts of other metal. The amount of photosensitive material present in the image areas is preferably between about 0.00l and about 10 grams per square meter and more preferably between 0.001 and 2 grams per square meter. The amount of binder is preferably between about 0.002 and about 30 grams per square meter and more preferably between about 0.001 and about 6 grams per square meter. The amount of tin present in the image areas varies from an amount necessary to obtain a visible image and can go up to l5 grams per square meter or greater. A preferred range for conductive tin images is from about 1.0 to about l5 grams per square meter. The tin metal forming the image is preferably finely divided for most photographic applications.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is particularly effective with photographic media the photosensitive component of which comprises a photoconductor which includes both organic and inorganic compounds and mixtures of compounds. Preferred photoconductors are inorganic compounds such as compounds of a metal and a nonmetallic element of Group VIA of the Periodic Table*, for example metal oxides, such as zinc oxide, titanium dioxide. antimony trioxide, aluminum oxide, zirconium dioxide germanium dioxide, indium, stannic oxide, barium titanate, lead oxide, tantalum oxide, and tellerium oxide; metal sulfides such as cadmium sulfide, zinc sulfide and stannic sulfide; and metal selenides, such as cadmium selenide. Metal oxides are generally preferred, and, of these, titanium dioxide, because of the unusually good results obtained therewith. Periodic Table from Lange's HANDBOOK OF CHEMISTRY, 9th Edition. pp. 56-57, I956.

A simple test useful in determining whether a selected material has a photoconductor effect involves mixing the test material with an aqueous solution of silver nitrate. Little, if any, reaction should take place in the absence of light. On subjecting the test mixture to light such as ultraviolet light along with a control solution of silver nitrate, the rate of darkening of the test solution compared to the control solution is determined; if faster than the control solution, the test material is a photoconductor.

The photographic media comprise the photoconductor on an inert carrier sheet which comprise any suitable backing of sufficient strength and durability to satisfactorily serve as a reproduction carrier. The carrier sheet may be in any form such as, for example, sheets, ribbons, rolls, etc. The sheet can be made of any suitable material such as wood, rag content paper, pulp paper, plastics, e.g. polyethylene terephthalate and cellulose acetate, cloth, metallic foil such as aluminum foil, and glass. The preferred form of the carrier sheet is a thin sheet which is durable and flexible.

The photoconductor is normally applied to the carrier with a binding agent. In general, these binders are translucent or transparent so that they do not interfere with transmission of light therethrough. Preferred binder materials are organic materials such a resins, e.g. butadienestyrene copolymers, poly (alkylacrylates) such as poly( methyl methacrylate), polyamides, polyvinyl acetate, polyvinyl alcohol and polyvinylpyrrolidones.

Other photographic media containing other photosensitive components than the aforementioned which are developed with physical developers are useful in this invention, for example, systems in which the photosensitive component is a diazonium salt or a diazosul- Fonate as described in the US. Pat. Nos. 2,l83,447 and 2,750,292, respectively.

The exposure of the photographic medium can be by my of the art-recognized procedures such as described n the US. patents and British specification mentioned ierein.

The period of exposure will depend upon the intenity of the light source, the particular imaging material, iarticular photoconductor, the type and amount of catalyst, if any, and like factors known to the art. In general, however, the exposure may vary from photo-flash time up to several minutes.

The latent metal image formation is accomplished by contacting the activated medium with a redox system composed of an oxidizing agent optionally containing a reducing agent. The oxidizing agent is usually silver ions, but also includes such metal ions as mercury, copper, gold, tin or palladium ions and thus is the imageforming component of the image-forming material. The reducing agent of the redox system can be any of the known reducing agents for the oxidizing agent which are compatible with the present media and systems. For example, the reducing agents include organic compounds such as oxalates, formates, substituted and unsubstituted hydroxylamine, substituted and unsubstituted hydrazine. ascorbic acid, aminophenols, diamines and dihydric phenols. Specific suitable reducing agents include hydroquinone, oand p-aminophenol, pmethylaminophenol, p-hydroxyphenylglycine, oand p-phenylenediamine and l-phenyl-3-pyrazolidone. The formation of the latent image is a function of the concentration of oxidizing agent, i.e. metal ion, and, if used, the activity of the reducing agent. The more facile method of controlling the extent of latent image is by controlling the quantity of metal ion in the medium prior to reaction with the reducing agent. Such considerations are well within the skill of the art and should not require excessive explanation herein. It should suffice for the purpose of this disclosure to indicate that the procedure for preparing the latent metal images is accomplished by controlling the amount of metal ion in the photographic medium by exposing to extremely dilute solutions of the metal ion or controlling the length of time of immersion of the medium in a solution of metal ion of higher concentration. Although either procedure can be used with equal effectiveness, it is preferred to utilize dilute solutions of the metal ion, particularly where the metal is silver, in view of economic considerations. As is obvious to any one in the art, the metal ion can be provided in the form of a soluble compound of the metal which does not adversely affect the desired effect. For example, silver ion is provided by dissolving silver nitrate in water or methanol.

The exposed photographic medium once sensitized with the oxidizing agent, i.e. metal ion, can then be treated with the reducing agnet, e.g. in solution which is generally stabilized to permit longer shelf-life. The most commonly used stabilizer is sodium sulfite although many other stabilizers are available and known to those skilled in photographic processing. This treatment is by the standard methods and does not require any special methods beyond those normally exercised in routine photographic processing. Optimum conditions for this step are easily determinable and are dependent on the selected reducing agent and the specific metal ion. When using silver ion as the oxidizing agent, p-methylaminophenol is the preferred reducing agent since it appears to give the best overall results and the reducing conditions are quite compatible with the remaining processing steps, i.e. the image intensification or amplification with tin.

Before proceeding with the image intensification, it is usually desirable, but not always essential, to fix the media to remove traces of metal ion which would be reduced in the subsequent processing steps. lf the initial metal ion solution is sufficiently dilute, the fixing step is not always necessary. When significant amounts of the metal ion are present in the media, however, they should be removed by any of the art-recognized methods. For example. when the metal ion is silver, the preferred metal at present, a solubilizing agent can be used. Usually, the most facile method entails the use of agents which form soluble complexes with silver ion, such as thiosulfate or thiocyanate ion, the former being preferred under most circumstances. In lieu of a separate fixing step, the solubilizing agent, e.g. thiosulfate ion. can be incorporated into the subsequent treatment solution. e.g. the stannous ion-containing solution, or even in the chromous ion reducing solution, or in both solutions. as desired.

The image intensifying systems of the present invention comprise a solution of stannous ions and a solution of chromous ions. The stannous ion solution may be prepared with any soluble stannous salt, e.g. the chlo' ride or sulfate salts, which will provide stannous ion in solution. As is well known, stannous salts tend to hydrolyze in aqueous systems to form precipitates and are subject to ready oxidation to stannic ions in solution. To minimize these possible reactions, it is preferred to complex the stannous ion in solution by formation of soluble, stable complexes, preferably anionic complexes such as the wellknown halide complexes. The tendency to hydrolyze or oxidize is minimized when the stannous ion is in complex ion form and it is preferred to use this form in the stannous solutions of the present amplification system. The preparation of such solutions is within the skill of the art and should require little, if any, elaboration. If desired, the stannous ion solution can be formed by reduction of stannic ions in solution by art-recognized methods.

Especially effective stannous complexes are those formed with halide ions, especially the chloro com plexes. Such complexes are prepared by adding a source of chloride ions, usually at high molar excesses, to a solution of stannous ions and the complex forms in solution. Effective sources of excess chloride ion are ammonium chloride, alkali metal chlorides, especially sodium or potassium chloride, and hydrochloric acid. The stannous ion solution appears to function best in the present amplification system when the ratio of ex cess chloride ions is from about 2 to about moles per mole of stannous ion. In this molar ratio, the preferred is from about 4 to 8 since the resulting amplified photographic images show lowest fog levels when this range is used, optimum results being obtained at the higher molar ratios.

The extent of amplification of the latent metal image is also influenced by the presence of strong acids in the stannous ion solution. Small quantities of such acids as hydrochloric and sulfuric acid apparently enhance the density and contrast of the amplified image when added to the stannous solution. For example, with the preferred molar ratio, as little as several milliliters of concentrated sulfuric acid or hydrochloric acid per liter of stannous solution appreciably enhances the density and quality of the amplified images.

As will be fully appreciated, the concentration of stannous ion in the solution is not excessively critical. A minimum of routine experimentation will indicate the optimum concentration of stannous ion for any given system. Usually, the concentration will be found in the range from about 0. [M to about 1.0M, and preferably from about 0.25M to about 0.6M, although other concentrations can also be used but without any appreciable benefit, and possibly with some difficulty, particularly with more concentrated solutions.

The solution of reducing agent can be prepared by disolving the reducing agent in a suitable solvent, which, for practical purposes, is normally an aqueous solvent system, and usually water to which may be added other compatible solvents such as lower alkanols. For most purposes, water suffices as solvent and is preferred. The addition of other solvents compatible with the present system is generally avoided but tolerable within practical limits as should be obvious to anyone skilled in the art. More conveniently, the reducing agent is formed, in situ, by reduction of a suitable solution of a source of chromic ions. The reason for preference should be obvious to those skilled in the art and is primarily predicated on the instability of chromous ions in solution due to the ease of oxidation to chromic ions under ambient conditions. Many methods are available for effecting reduction of chromic to chromous ions and one such is the method described in Handbook of Preparative Inorganic fihemistry Vol. 2, p 1367, Second Edition, Academic Press, NY. (1965). which, at present, is preferred. Such a method is also preferred since the reducing agent solution can be regenerated readily by merely effecting reduction of chromic ions to the desired chromous ions and the initial chromous ion solution can be regenerated after being partially or even completely spent in use. The chromous ion solution generally contains a strong acid, such as sulfuric acid, for best results.

The concentration of the reducing agent should be sufficient to accomplish the desired reduction of stannous ions. For any given system, the optimum concentration can be readily determined by a minimum of experimentation. While a wide range of concentration can be employed, in general, the optimum concentration of reducing agent is in the range of from about 0.1M to about 0.5M. As should be obvious to those skilled in the art, the concentration of reducing agent is not critical, except as a matter of efficiency of operation and the time requirements of any processing sequence. The aforementioned preferred range of concentration appears to given the most desirable results from the viewpoint of effective image amplification in relatively short periods of time. For example, when amplifying a latent silver image with stannous ion, the aforementioned preferred concentration results in full amplification within about 1 minute generally reaching a maximum within about 30 seconds and often in shorter time periods. Other concentrations are operable with suitable alteration in operating time periods.

In general, the intensification system of this invention comprises a two solution system containing stannous ions and reducing agent in the respective solutions as previously described. The process of fully developing a photoexposed medium comprising a radiationactivatable photoconductor can be in a three solution system as follows:

1. oxidizing agent, e.g. silver ion solution 2. stannous ion solution 3. stannous ion-reducing agent solution.

The inclusion of a reducing agent for the oxidizing agent can be effected by provision of a separate solution to be used after solution 1 and before solution 2, or the reducing agent can be included in solution 2. Each of the systems, i.e. 3 solutions and 4 solutions,

makes the overall processing eminently suited for automated processing, for example, in automated photographic development apparatus, or in photoduplicating apparatus which utilize copy media as hereindescribed for multiple copying. The three solution system is particularly suited for automated processing, particularly in the production of multiple copies of a master. The three solution system permits the use of a developing system composed on only three treatment stations, i.e. baths, in the automatic processor which has additionally the very desirable advantage of being based on only slight silver requirements, the visible images being predominantly tin. For example, the silver bath can be of a concentration of as little as 0.00l M silver ion and even lower. lf tin intensification is not used, the normal requirement of silver ion is at a concentration of about 0.2M to obtain a black image. The economic advantage of replacing silver in the image formation becomes more apparent when it is realized that one pound of silver nitrate will make about 700 gallons of 0.00lM silver nitrate, but only 3.5 gallons of 0.2M solution.

The overall processing time is also an outstanding advantage of the present intensification systems, processing times of l-2 minutes being quite practical with the aforementioned three solution development system.

Additionally, by having the tin ions separate from the source of chromous ions, applicant has achieved a sys tem having very stable developer baths which can be readily used many times and over a long period of time without serious degradation. These solutions can be stored almost indefinitely when not being used by stor ing in a non-oxidizing environment such as under an inert gas as argon or in a sealed container. This compares to a developer life when not being used for a unitary developer comprising both tin ions, chromous ions, and chromic ions and a stabilizing surfactant, such as mentioned in U.S. Pat. No. 3,637,386, ofa few hours or at the most a few days time before completely degrading even when stored in a non-oxidizing environment such as mentioned above.

Additionally, the chromous ion solution can easily be regenerated electrolytically or by utilizing a strong re ducing agent such as zinc metal. This is much simpler than the procedure required to regenerate a degraded bath containing a metal ion and a reducing agent which would require a number of precise additions ofjust the right components.

The quality of the intensified image can be controlled as to the blackness of the tin deposit as described in the literature on plating with tin, e.g. Metal Finishing Guidebook, l956 Edition Finishing Publications, Inc., 38l Broadway, Westwood, NJ. Phenolic compounds such as cresol phenol, B-naphthol and hydroxyaryl sulfones, sulfoxides and sulfonates and similar additives :an be added to either the stannous ion solution or, )referably, the reducing agent solution, preferably at a :oncentration ranging up to IO M.

The use of glue, especially animal glue, in the chro nous ion solution enhances the formation of the ampliied image. Generally, minor amounts of the additives ire all that appears required to obtain optimum quality. or example, animal glue is employed at levels as little .5 1 gram per liter of chromous solution while cresol is Ised at less than l gram per liter of chromous solution.

In the foregoing description, and in the following exmples, the preferred order of use of the stannous ion olution and the reducing agent is the sequence indicated. However, it is possible to first immerse the latent metal image-bearing medium in the reducing agent, followed by the stannous ion solution. However, with this reverse order, the results obtained may not always be optimum, nor easily reproduceable, for which reason, though operable, it is not preferred.

The present amplifying systems can also be used to amplify latent images in photosensitive media other than those containing a reversibly activable photoconductor such as titanium dioxide as the photosensitive component. For example, a photoconductor such as silver halide in a silver halide emulsion film is especially desirable. Weak latent images can be intensified using the present amplifying systems, in general, using the same procedures hereinbefore described.

The following examples are intended to further illustrate the present invention.

In the following examples, the chromous ion solution was prepared by reduction with zinc by the following procedure:

A glass tube approximately 25 mm. in diameter and about l25 mm. long fitted with a glass frit and a stopcock on one end is filled approximately two-thirds full with granulated zinc. The zinc is then treated for 10 minutes with a solution which is 0.lM mercuric chloride and 1.0M hydrochloric acid, following which the column is washed with a large amount of water and finally with 1.0M sulfuric acid. A solution of chromic chloride in 1.0M H SO is then passed through this col umn at a rate such that the solution collected in a re ceiver under an argon atmosphere is the characteristic blue color of chromous ion solution with no evidence of the green color indicating unreduced chromic chlo ride solution. The sulfuric acid of the chromic chloride solution can be replaced by other acids such as hydrochloric acid. The following examples further illustrate the invention described herein.

EXAMPLE l A titanium dioxide coated paper is exposed through a line negative on an E. G. & G. sensitometer l0 second exposure), held for l0 seconds and then is pro cessed as follows: A.

l. immerse for 10 seconds in 0.005 M AgNO and drain 10 seconds 2. immerse for 10 seconds in a silver developer (Zg/l.

p-methylaminophenol and 7.5 g/l. Na SO containing 50 g/l. sodium thiosulfate) and drain 5 seconds. 3. immerse for 30 seconds in 0.5M Sncl which is also 2.5M Nacl and contains 16 ml. conc. hydrochloric acid per liter and drain 10 seconds 4. immerse for 20 seconds in 0.25M CrCl which is also lM. H 80, and water wash to obtain a clear image of good contrast and density. B. The procedure of (A) is repeated with 5ml. of 1M trisodium nitrilotriacetate added per liter of 0.005M

The contrast and density are better than the image obtained with procedure A.

EXAMPLE 2 Paper sheets coated with titanium dioxide in gelatin are exposed to a line negative for 3 seconds on an E.G. & G. sensitometer with a mercury-vapor lamp and held for 10 seconds. The sheets are then processed through the same steps as procedure B of Example 1 to obtain a clear print of the line negative.

EXAMPLE 3 Silver primary images are developed in stepwedge exposures prepared as in procedure A of Example 1 and are amplified with stannous ion as in the same procedure A at varying concentrations of NaCl and H 50 The results are given in the following tabulation which shows the increased number of steps obtained over the original silver primary image SnCl NaCl H2504 Increased No.

(mL/l.) of steps 0.5M 1.5M 25 l 11.5 1-2 22.5 1-2 32.5 O-l 0.5M 2.0M 1.6 2-4 5.6 2 5M [.6 1-2 4.0 0.1 8.0 2-3 15 1-2 28 1-2 3.0M 0.4 0-1 2 l-2 4 2 6 2-3 t0 1 21 t 31 0-l 3.5M 1.2 2-3 3 2-3 4 2 B 3 12 3 I6 2 From these results, it is apparent that lower sulfuric acid concentration is required as the sodium chloride concentration is increased to obtain best results, i.e. maximum number of steps and minimum fog levels. In general. the samples with the highest number of steps also have the lowest fog levels. For example, the sample processed with 3.5M NaCl and 12 ml. of H 80, shows a fog level of about 0.3, whereas the average fog level of the other samples is about 0.4.

EXAMPLE 4 A B C D Increased No. of steps 2 2 3 2-3 D Max .88 .90 .86 .90 fog .37 .32 .32 .29

EXAMPLE 5 When hydrochlorlc acid l5 substituted for sulfuric acid in the procedure of Example 3. the results are somewhat better since lower fog levels are obtained,

the average fog level being about 0.3 as contrasted with the average of 0.4 for sulfuric acid. In most instances, the increase in the number of steps is greater with hydrochloric acid. The results are tabulated below:

SnCl NaCl HCl Increased no. fog level m1/l.added of steps 0.5M 3.0M 2.4 3-4 .28 5.0 34 .25 8.0 3-4 .30 12.0 1-2 .29 16.0 1-2 .31 2.5M 08 2-3 .44 4.0 2-3 .33 6.0 2-3 .32 8.0 3 .28 12.0 2-3 .24 16.0 2-3 .33 3.5M 5.0 0 .46 8.0 3-4 .28 12 1-3 .35

EXAMPLE 6 Titanium dioxide coated sheets are exposed on a sensitometer as in Example 2 and are processed in the following manner:

A. immerse for 10 seconds in l0"M. AgNO solution containing 2.5 ml. of 1M trisodium nitrilotriacetate and drain 10 seconds.

B. immerse for 30 seconds in silver developer (described in Example 1) and drain 10 seconds C. immerse for 30 seconds in 0.5M SnCl; in 3.0M

Nacl 8 ml. Hcl/l. and drain 10 seconds.

D. immerse for 30 seconds in 0.5M CrCl, in 1M. H SO containing 2g/1. glue and 0.3g/l cresol and wash for 10 minutes. The stepwedge image shows 18 steps, Dmax 0.74 and fog level of 0.27.

When the immersion time in the chromous chloride solution is increased to 40 seconds, the density increased.

EXAMPLE 7 Titanium dioxide coated sheets are exposed through a stepwedge on the sensitometer for 10' seconds, held for 10 seconds and treated in the following sequence:

1. immerse for 10 seconds in 0001M AgNO and drain 10 seconds.

2. immerse for 30 seconds in silver developer described in Example 1 and drain 10 seconds.

3. immersefor 30 seconds in 0.5M Sncl, in 3.0M

NaCl containing SmLHCl/liter and drain 10 secends 4. immerse for 40 seconds in 0.25M CrC1 in 1M H 50. and wash 10 minutes.

Trisodium nitrilotriacetate (Na NTA) is added to each of the first two treatment solutions (25 ml and 50 ml. respectively of 1M.Na NTA per liter). The amplifled image shows 18 steps, Dmax 0.71, and fog level of 0.23.

When animal glue 1 g/l.) is added to the Crcl solu tion, the fog level reduced to 0.21; at 2g/1., to 0.19, with an increase of 1 step.

EXAMPLE 8 Titanium dioxide-coated sheets are exposed on a sensitometer for 10' seconds, held for 10 seconds, and processed according to the following sequence:

1. immerse for 10 seconds in 0.001 M AgNO, containing 25 m1./1. of 1M Na NTA and drain 10 seconds.

2. immerse for 30 seconds in silver developer (described in Example 1) containing SOmL/l. of 1M Na NTA and drain 10 seconds.

3. immerse for 30 seconds in 0.5MSnCl in 3.0M NaCl containing 5ml./l. HCl and 25"ml./l. lM. Na NTA and drain seconds.

4. immerse for 40 seconds in 0.375 M. Crcl in lM H SO containing 3g/l animal glue and 0.5g/l. cresol and wash for 10 minutes. The stepwedge print shows 19 steps, Dmax 0.73 and fog level, 0.23.

In the foregoing examples, the photosensitive media are principally paper and plastic film coated with the photoconductor. Similar results are obtained with metal foil coated with titanium dioxide, such as aluminum sheets coated with titanium dioxide. e.g. as described in commonly assigned copending U.S. applica tion Ser. No. 446,707 filed Apr. 8, 1965, now abandoned.

In addition to amplifying photographic images as described in the foregoing examples, the present systems and processes are also useful in the production of printed circuits as described in commonly assigned copending U.S. applications Ser. No. 721,778, filed Apr. 16, I968 and 717,502, filed Apr. 1, 1968 now abandoned.

In the foregoing description and in the appended claims, reference is made to a source of stannous ions and a source of chromous ions, or solutions of the respective ions. As will be appreciated by those skilled in the art, such sources, and solutions, should contain only ions compatible with the specified processes and should not contain ions or substances which adversely affect or prevent the desired result, i.e. the sources, and solutions, should be photographically-acceptable. The selection of suitable sources of the respective ions should take into account this aspect. Further, the selection of the sources of ions should also avoid the formation during processing of undesirable precipitates which are avoided in photographic developing for obvious reasons.

The amplified visible images produced in accordance with the foregoing examples are images of finely divided metal, the metal being substantially tin. Of course, the image also includes the metal of the latent metal image, e.g. copper or silver, which is present in minor amount, usually less than 10% by weight of the total metal image, and preferably less than 5% by weight when the metal is silver, for economic reasons. The visible tin metal images are especially stable and resist degradation, e.g. oxidative degradation, under ambient conditions of storage as well as accelerated shelf-life tests, e.g. extreme conditions of high temperature and relative humidity which are generally used in the photographic art to determine shelf-life. For example, such a tin-amplified photographic image is stored in an atmosphere of high relative humidity and a temperature of about F. for 1 month with no appreciable change in the image density. Obviously, the tin image is of a high order of stability which makes the present process of considerable importance to the art in providing permanent amplified photographic images. Comparison of tin-amplified photographic images with copper-amplified photographic images shows that the tin images are of a higher order of stability, particularly in accelerated shelf-life tests.

I claim:

1. Process of amplifying a metal image on a photographic medium which image is capable of initiating deposition of free tin when contacted with a physical developer comprising a solution of stannous ions, and which comprises contacting said medium with a source of stannous ions and then in a separate step contacting the medium with a source of chromous ions as reducing agent for said stannous ions.

2. Process as in claim 1 wherein said photographic medium comprises a photosensitive silver halide.

3. Process as in claim 1 wherein the source of stannous ion is an anionic complex thereof.

4. Process as in claim 3 wherein the complex is a chloro complex.

5. Process as in claim 3 wherein said source includes a strong acid.

6. Process as in claim 1 wherein the source of chromous ion is a chromous salt of a strong acid.

7. Process as in claim 6 wherein the said source includes a strong acid.

8. Process of image recording comprising:

l. forming a metal image on a copy medium comprising photosensitive titanium dioxide by a process including an imagewise exposure step, said metal image being capable of initiating deposition of free tin when contacted with a physical developer comprising a solution of stannous ions;

2. contacting this imaged medium with a source of stannous ions; and then in a separate step 3. contacting this medium with a source of chromous ions which reduces the stannous ions to free tin in areas corresponding to said metal image.

9. Process of producing a visible image on an imagewise-exposed photographic medium comprising titanium dioxide which comprises contacting said medium with a source of silver ions to form a latent metal image which image is capable of initiating deposition of free tin when contacted with a physical developer comprising a solution of stannous ions, and amplifying said image by contacting with a source of stannous ions and then in a separate step a source of chromous ions as reducing agent for said stannous ions.

I I k 

1. PROCESS OF AMPLIFYING A METAL IMAGE OF A PHOTOGRAPHIC MEDIUM WHICH IMAGE IS CAPABLE OF INITIATING DEPOSITION OF FREE TIN WHEN CONTACTED WITH A PHYSICAL DEVELOPER COMPRISING A SOLUTION OF STANNOUS IONS, AND WHICH COMPRISES CONTACTING SAID MEDIUM WITH A SOURCE OF STANNOUS IONS AND THEN IN A SEPARATE STEP CONTACTING THE MEDIUM WITH A SOURCE OF CHROMOUS IONS AS REDUCING AGENT FOR SAID STANNOUS IONS.
 2. Process as in claim 1 wherein said photographic medium comprises a photosensitive silver halide.
 2. contacting this imaged medium with a source of stannous ions; and then in A separate step
 3. contacting this medium with a source of chromous ions which reduces the stannous ions to free tin in areas corresponding to said metal image.
 3. Process as in claim 1 wherein the source of stannous ion is an anionic complex thereof.
 4. Process as in claim 3 wherein the complex is a chloro complex.
 5. Process as in claim 3 wherein said source includes a strong acid.
 6. Process as in claim 1 wherein the source of chromous ion is a chromous salt of a strong acid.
 7. Process as in claim 6 wherein the said source includes a strong acid.
 8. Process of image recording comprising:
 9. Process of producing a visible image on an imagewise-exposed photographic medium comprising titanium dioxide which comprises contacting said medium with a source of silver ions to form a latent metal image which image is capable of initiating deposition of free tin when contacted with a physical developer comprising a solution of stannous ions, and amplifying said image by contacting with a source of stannous ions and then in a separate step a source of chromous ions as reducing agent for said stannous ions. 